I have made way more money from my bitcoin capital gains than from anything else I did for the last 3 years.
I don't know why the original post got down-voted. Crytocurrency is still in its infancy. Much money can be made from it. And it definitely qualifies as "passive".
Now, everyone (ie professional investors) knows about cryptocurrency so you have no advantage over them and you're just gambling.
Here's a strategy that works: Take an amount of money that you own but can miss. This means that you won't go totally crazy when you lose it all. Put that amount in Bitoin, Ethereum and Monero for example. Let it sit there for the next 10 years.
In 10 years, it's either $0 and you lost it all, or it became the best investment you ever did. And that is counted in factors, not percentages.
Worked out so far.